In order to avoid damage to the harvesting machine or spoilage of the harvested material by foreign objects such as stones or other hard objects in the crop flow it is known to provide a harvesting machine with a foreign object detection device so that the harvesting machine upon detection of such a device can automatically take the required actions to avoid the foreign object to be processed along with the crop material. Detection devices that detect foreign objects based on the effects they cause on a feeder assembly of a harvester have the advantage that upon detection near the feeder assembly the crop flow can be interrupted before the foreign object is further processed along with the crop material by the harvesting machine. However in order to successfully avoid processing of the foreign object the detection of it must occur as quickly as possible, as with harvesting machines with increasing productivity the time available for interrupting the crop flow in the feeder assembly before reaching subsequent processing elements is constantly decreased. Furthermore the detection device must operate as reliable as possible being able to detect an as wide range of foreign objects as possible while avoiding false detections interrupting the harvesting operation unnecessarily.
One such a detection device known from US2003/0115846. There the acceleration of the top rolls of the feeder assembly as a consequence of a foreign object in the crop flow is monitored. However because of the large inertia associated with this acceleration of the top rolls, the detection lags the occurrence of a foreign object considerably and only foreign objects that are large enough to cause a movement are detected. Furthermore lumps of crop material could equally cause similar accelerations top rolls leading to false detections.
U.S. Pat. No. 5,092,818 describes a detection device that acoustically detects airborne noise generated by an impact of a foreign object on the feed roll by means of a microphone. However because of the noise generated by several elements of the harvesting machine in operation the microphone needs to be arranged in close vicinity of the impact location and be provided with acoustical isolators. In practice reliable detection is difficult and extensive filtering of the signal is required which leads to detection lags.
It is further known from U.S. Pat. No. 4,353,199 to arrange a piezoelectric sensor on the feed roll itself. However the inductive transmission needed for the sensor signal from the sensor that is rotating with the feed roll during operation to a stationary cable by means of an inductive coupling leads to serious degradation of the signal quality and the risk of inducing disturbances to the signal. Furthermore this system also requires acoustic isolation by means of shock mounts at the height of the bearings of the feed roll on the central shaft. With heavily loaded feed rolls shock mounts at this location affect the stability of the feed roll assembly.
Still a further system is known from DE102008054488. This system proposes to arrange a piezoelectric vibration sensor on the central shaft of the feed roll such that it detects vibrations along the rotational axis of this central shaft. The subsequent processing requires filtering operations that induce detection lags which become critical for high capacity harvesting machines.
Therefor there exists a need for an improved detection device that overcomes the abovementioned disadvantages, increasing the reliability of detection of a foreign object while decreasing the detection lag, without requiring acoustic isolation.